However, if you try to overlay incompatible plot types, you will get an error message that saysERROR: Attempting to overlay incompatible plot or chart types.
For example, a histogram and a series plots are not compatible in PROC SGPLOT, so you need to use the Graphics Template Language (GTL) to overlay a custom density estimate on a histogram.

A similar limitation exists for bar charts in PROC SGPLOT: you cannot specify the VBAR and SERIES statements in a single call. However, in SAS 9.4m3 you can overlay a curve and a bar chart by using the new the VBARBASIC and the HBARBASIC statements. These statements create a
bar chart that is compatible with basic plots such as scatter plots, series plots, and box plots.

Overlay a curve on a bar chart in SAS

In most situations it doesn't make sense to overlay a continuous curve on a discrete bar chart, which is why the SG routines have the concept of compatible plot types. However, there is a canonical example in elementary statistics that combines continuous and discrete data: the normal approximation to the binomial distribution.

Recall that if X is the number of successes in n independent trials for which the probability of success is p, then X is binomially distributed: X ~ Binom(n, p). A well-known rule says that if np > 5 and n(1-p) > 5, then the binomial distribution is approximated by a normal distribution with mean np and standard deviation sqrt(np(1-p)).

This rule is often illustrated by overlaying the continuous normal PDF on a bar chart that shows the binomial distribution, as shown to the left.
To create this plot, I used the VBARBASIC statement to create the bar chart. Because the VBARBASIC statement creates a "basic plot," you can combine it with another basic plot, such as the line plot created by using a SERIES statement. For fun, I used an INSET statement to overlay a box of parameter values for the graph.
The graph shows that the binomial probability at j is approximated by the area under the normal density curve on the interval [j-0.5, j+0.5].

The following SAS statements use the PDF function to evaluate the binomial probabilities and the normal density for the graph.
The values for μ and σ are stored in macro variables for later use.

The preceding DATA step evaluates the Binom(15, 0.25) probability for the integers j=0, 1, ..., 14.
It evaluates the N(6.25, 2.17) PDF on the interval [-1.3, 13.8]. The following call to PROC SGPLOT uses the VBARBASIC statement to overlay the bar chart and the density curve:

The TYPE=LINEAR option on the XAXIS statement tells the horizontal axis to use interval tick marks.
The BARWIDTH=1 option on the VBARBASIC statement makes the bar chart look more like a histogram by eliminating the gaps between bars. The graph is shown at the top of this section.

Alternative visualization: The needle plot

If you are content to show only the height of the binomial probability mass function (PMF), you can use
an alternative visualization. The following graph shows a needle plot (the binomial PMF) overlaid with a normal PDF.
This visualization does not require 9.4M3. The SGPLOT statements are the same as before, except the binomial probabilities are represented by using the NEEDLE statement: needle x=j y=Binomial / markers;